CN103515541A - Substrate for OLED and method of manufacturing the same - Google Patents

Abstract

A substrate for an organic light emitting device (OLED) in which light extraction efficiency and processing efficiency of the OLED can be improved, and a method for manufacturing the same. The substrate for OLED includes a base substrate, a first metal oxide thin film coating one surface of the base substrate, a second metal oxide thin film coating the other surface of the base substrate, and coating the base substrate. A third metal oxide film on the surface of the second metal oxide film, the first metal oxide film having a first texture on its surface.

Description

Substrate for OLED and its manufacturing method

Cross References to Related Applications

This application claims priority from Korean Patent Application No. 10-2012-0067315 filed on Jun. 22, 2012, the entire contents of which are hereby incorporated by reference for all purposes.

technical field

The present application relates to a substrate for an organic light emitting device (OLED) and its manufacturing method, and more particularly, to a substrate for an OLED and its manufacturing method, in which light extraction efficiency and processing efficiency of the OLED can be improved.

Background technique

Generally, an organic light emitting device (OLED) includes an anode, a light emitting layer, and a cathode. When a voltage is applied between the anode and the cathode, holes are injected from the anode into the hole injection layer, and then migrate from the hole injection layer to the organic light-emitting layer, and electrons are injected from the cathode into the electron injection layer, and then migrate from the electron injection layer to the organic light-emitting layer. luminous layer. The holes and electrons that have migrated into the light emitting layer recombine with each other in the light emitting layer, thereby generating excitons. When such an exciton transitions from an excited state to a ground state, light is emitted.

Organic light emitting displays including OLEDs are classified into a passive matrix type and an active matrix type according to a mechanism of driving an N×M number of pixels arranged in a matrix shape.

In the active matrix type, a pixel electrode that determines a light emitting area and a unit pixel driving circuit that applies current or voltage to the pixel electrode are provided in a unit pixel area. The unit pixel driving circuit has at least two thin film transistors (TFTs) and a capacitor. Due to this configuration, the unit pixel drive circuit can supply a constant current regardless of the number of pixels, thereby achieving uniform luminance. The active matrix type organic light emitting display consumes a small amount of power and thus can be effectively used in high resolution displays and large displays.

However, only about 20% of the light generated by the OLED is emitted to the outside, and about 80% of the light is due to the wavelength effect generated by the difference in refractive index between the glass substrate and the organic light-emitting layer and caused by the refractive index difference between the glass substrate and the air. The organic light emitting layer includes an anode, a hole injection layer, a hole carrier layer, a light emitting layer, an electron carrier layer and an electron injection layer.

To overcome this problem, OLEDs are provided with optically functional layers such as light extraction layers and transparent conductive oxide thin film layers. In the prior art, such an optical functional layer is formed by photolithography. However, there are problems in that the cost increases due to the use of an expensive device, and since several optical functional layers are manufactured by different processes, the overall process becomes complicated, the processing time increases, and the manufacturing cost is increased. In addition, the light extraction layer formed by photolithography has problems such as weak adhesion to a substrate, and insufficient durability. In addition, in the prior art, because ITO is used as a transparent conductive oxide thin film layer, the manufacturing cost is increased.

The information disclosed in the Background of the Invention section is only for better understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms the prior art that is already known to a person of ordinary skill in the art.

Contents of the invention

Aspects of the present invention provide a substrate for an organic light emitting device (OLED) and a manufacturing method thereof, wherein light extraction efficiency and processing efficiency of the OLED are improved.

In one aspect of the present invention, a substrate for an OLED is provided, comprising: a base substrate; a first metal oxide thin film coating one surface of the base substrate, the first metal oxide thin film on its surface having a first texture; a second metal oxide thin film coating the other surface of the base substrate; and a third metal oxide thin film coating a surface of the second metal oxide thin film.

According to an embodiment of the present invention, the first metal oxide thin film may be an external light extraction layer of the OLED, the second metal oxide thin film may be an internal light extraction layer of the OLED, and the third The metal oxide film can be the transparent electrode of the OLED.

The second metal oxide thin film may have a second texture on its surface, wherein the surface is coated with the third metal oxide thin film.

The substrate for OLED may further include a planarization layer between the second metal oxide thin film and the third metal oxide thin film.

A surface of the second metal oxide thin film coated with the third metal oxide thin film may form a flat surface.

Each of the first to third metal oxide thin films may include a metal oxide, a solid solution of at least two metal oxides, or a multilayer structure of at least two metal oxides, wherein the metal oxide is selected from In the group consisting of ZnO, SnO ₂ , SiO ₂ , Al ₂ O ₃ and TiO ₂ .

The haze value of the outer light extraction layer may be 60% or more, the haze value of the inner light extraction layer may be 5% or more, and the haze value of the transparent electrode may be 10% or more. smaller.

In addition, the sheet resistance of the transparent electrode may be 15Ω/□ or less.

In the range of visible light, the transmittance of the outer light extraction layer may be 40% or more, the transmittance of the inner light extraction layer may be 50% or more, and the transmittance of the transparent electrode may be 70% or greater.

In addition, the refractive index of the outer light extraction layer may be in the range of 1.4 to 3.0, the refractive index of the inner light extraction layer may be in the range of 1.4 to 3.0, and the refractive index of the transparent electrode may be in the range of 1.7 to 3.0. within range.

In another aspect of the present invention, there is provided a method of manufacturing a substrate for an OLED, the method comprising depositing at least one layer on each of one surface and the other surface of a base substrate by atmospheric pressure chemical vapor deposition (APCVD). metal oxide films.

According to an embodiment of the present invention, the step of depositing at least one metal oxide thin film may include: depositing a first metal oxide thin film on one surface of the base substrate as an external light extraction layer of the OLED; depositing a second metal oxide thin film on the other surface of the base substrate as an internal light extraction layer of the OLED; and depositing a third metal oxide thin film on the surface of the second metal oxide thin film as an inner light extraction layer of the OLED transparent electrodes.

The step of depositing the second metal oxide thin film and the step of depositing the third metal oxide thin film may be performed after the step of depositing the first metal oxide thin film. Alternatively, the step of depositing the first metal oxide thin film may be performed after the step of depositing the second metal oxide thin film and the step of depositing the third metal oxide thin film.

The method may further include forming a planarization layer on the second metal oxide thin film between the steps of depositing the second metal oxide thin film and depositing the third metal oxide thin film.

The step of depositing the third metal oxide thin film may include doping the third metal oxide with at least one n-type dopant including Ga, Al, F, Si, and B and a p-type dopant including N. object film.

The step of depositing the first metal oxide thin film, the step of depositing the second metal oxide thin film, and the step of depositing the third metal oxide thin film may be performed by in-line processing.

In addition, each of the first to third metal oxide thin films may be formed with a metal oxide or a solid solution of at least two metal oxides, wherein the metal oxide is selected from ZnO, SnO ₂ , SiO ₂ , Al ₂ In the group consisting of O ₃ and TiO ₂ .

According to an embodiment of the present invention, since the external light extraction layer having a texture is formed on the front and rear surfaces of the substrate, light extraction efficiency of the OLED may be increased.

In addition, since the inner and outer light extraction layers and the transparent conductive metal oxide thin film of the OLED are fabricated sequentially by APCVD, the processing time can be reduced and the function matching ability can be improved.

In addition, since a metal oxide that is cheaper than ITO used in the related art is used to form the light extraction layer and the transparent conductive oxide thin film, the manufacturing cost can be reduced.

The accompanying drawings and the following detailed description of the invention, incorporated herein, may make other features and advantages of the method and apparatus of the invention more apparent or explain in more detail, which together explain certain principles of the invention.

Description of drawings

1 is a cross-sectional view showing a substrate for an organic light emitting device (OLED) according to an embodiment of the present invention;

2 and 3 are process views sequentially showing a method of manufacturing a substrate for an OLED according to an embodiment of the present invention; and

4 to 7 are Scanning Electron Microscope (SEM) images of cross-sections of a substrate for an OLED manufactured by a method of manufacturing a substrate for an OLED according to an embodiment of the present invention.

Detailed ways

Reference will now be made in detail to a substrate for an organic light emitting device (OLED) and its manufacturing method, the embodiments of which are illustrated in the accompanying drawings and described below so that those of ordinary skill in the art related to the present invention can easily Put the invention into practice.

Throughout, reference will be made to the drawings, wherein the same reference numerals and symbols are used throughout the different drawings to refer to the same or similar parts. In the following description of the present invention, a detailed description of known functions and elements incorporated herein will be omitted when it may make the subject matter of the present invention unclear.

As shown in FIG. 1 , a substrate 100 for an OLED according to an embodiment of the present invention is a substrate intended to improve light extraction efficiency of an OLED. The substrate 100 is bonded to one surface of the OLED as one of the substrates of a pair of OLEDs facing each other. The substrate 100 functions as a channel through which light generated by the OLED is emitted to the outside while protecting the OLED from external environment damage.

Although not shown, the OLED has a multilayer structure including an anode, an organic light emitting layer, and a cathode disposed between the substrate 100 according to this embodiment of the present invention and an encapsulation substrate opposite to the substrate 100 . The anode is formed as part of the substrate 100 according to this embodiment of the invention. This will be explained in more detail later. The cathode is realized as a metal thin film of Al, Al:Li or Mg:Ag with a low work function to facilitate electron injection. In the case of a top emission structure, the cathode may have a multilayer structure including a semitransparent electrode of a metal thin film of Al, Al:Li, or Mg:Ag and a transparent electrode of an oxide thin film of tin oxide (ITO). , to facilitate the transmission of light generated by the organic light-emitting layer. In addition, the organic light-emitting layer includes a hole injection layer, a hole carrier layer, a light-emitting layer, an electron carrier layer, and an electron injection layer sequentially stacked on the anode. In this structure, when a forward voltage is applied between the anode and the cathode, electrons from the cathode migrate to the light-emitting layer through the electron injection layer and the electron carrier layer, and holes from the anode pass through the hole injection layer and the hole carrier layer migrates to the emissive layer. The electrons and holes that have migrated into the light emitting layer recombine with each other, thereby generating excitons. When such excitons transition from an excited state to a ground state, light is emitted.

The substrate 100 for an OLED according to this embodiment of the present invention to be bonded to an OLED as described above includes a base substrate 110, a first metal oxide film 120, a second metal oxide film 130, and a third metal oxide film 140. . Here, the base substrate 110, the first metal oxide thin film 120, the second metal oxide thin film 130, and the third metal oxide thin film 140 are manufactured through an in-line process based on atmospheric pressure chemical vapor deposition (APCVD), thereby forming a package.

The base substrate 110 is a transparent substrate, which may be made of any material without limitation as long as it has good light transmittance and excellent mechanical properties. For example, the base substrate 110 may be made of a polymer material such as a thermally cured or ultraviolet (UV) cured organic film or a polymer material such as soda lime glass (SiO ₂ —CaO—Na ₂ O) or aluminosilicate glass (SiO ₂ —Al ₂ O ₃ -Manufacture of chemically tempered glass with Na ₂ O). The amount of Na can be adjusted according to the application. Here, when the OLED is used for lighting, soda lime glass may be used, and when the OLED is used for a display, aluminosilicate glass may be used.

According to an embodiment of the present invention, the base substrate 110 may be implemented as thin glass having a thickness of 1.5 mm or less. The thin glass is manufactured by fusion process or troweling process.

The first metal oxide thin film 120 is formed such that it coats one surface of the base substrate 110 . For example, the first metal oxide thin film 120 may coat the upper surface (relative to the paper surface) of the base substrate 110 . Here, it is preferable that the thickness of the first metal oxide thin film 120 coating the upper surface of the base substrate 110 is in the range of 0.2 to 5 μm. The first metal oxide thin film 120 may include a metal oxide, a solid solution of at least two metal oxides, or a multilayer structure of at least two metal oxides, wherein the metal oxide is selected from ZnO, SnO ₂ , SiO ₂ , Al ₂ O ₃ and TiO ₂ in the group.

In addition, as shown in FIGS. 1 , 4 and 5 , a first texture 120 a is formed on the surface of the first metal oxide film 120 . The first texture 120a serves to scatter light in the visible range and may be patterned so that it has the shape of a rod, a semi-hexagonal or hexagonal prism, or a random shape feature. When the first metal oxide thin film 120 is deposited by APCVD, the first texture 120a may be spontaneously formed. This will be explained in more detail later in relation to the method of manufacturing a substrate for an OLED.

As shown in the figure, the first metal oxide thin film 120 formed on the upper surface of the base substrate 110 is the outermost layer of the substrate 100 for OLED, and serves as an external light extraction layer of the OLED. The first metal oxide thin film 120 formed as an external light extraction layer in this way has a haze value of 60% or more, a transmittance of 40% or more in the range of visible light, and a range of 1.4 to 3.0. The inner refractive index.

The second metal oxide thin film 130 is formed such that it coats the other surface of the base substrate 110 , that is, the lower surface of the base substrate 110 facing the oxide thin film 120 (with respect to the paper surface). It is preferable that the thickness of the second metal oxide thin film 130 on the lower surface of the base substrate 110 is in the range of 0.2 to 5 μm. The second metal oxide film 130 may be made of the same material as the first metal oxide film 120 . Specifically, the second metal oxide film 130 may include a metal oxide, a solid solution of at least two metal oxides, or a multilayer structure of at least two metal oxides, wherein the metal oxide is selected from ZnO, SnO ₂ , In the group consisting of SiO ₂ , Al ₂ O ₃ and TiO ₂ .

As shown in FIGS. 1 and 6, the second metal oxide film 130 has a second texture 130a on one surface thereof. The second texture 130a serves to scatter light and can be patterned so that it has the shape of a rod, a rod with a half hexagon on one side of it, or a hexagonal pillar, or random shape features. A surface of the second metal oxide film 130 on which the second texture 130 a is formed is adjacent to the third metal oxide film 140 . Like the first texture 120a, when the second metal oxide thin film 130 is deposited by APCVD, the second texture 130a may be spontaneously formed. However, one surface of the second metal oxide thin film 130 may form a flat surface.

A planarization layer (not shown) may be formed on the surface of the second texture 130 a , that is, on the interface between the second metal oxide film 130 and the third metal oxide film 140 . A planarization layer (not shown) is formed in a later process and is intended to ensure planarity of the third metal oxide thin film 140 serving as a transparent electrode or anode of the OLED.

The second metal oxide film 130 may include a metal oxide, a solid solution of at least two metal oxides, or a multilayer structure of at least two metal oxides, wherein the metal oxide is selected from ZnO, SnO ₂ , SiO ₂ , In the group consisting of Al ₂ O ₃ and TiO ₂ . The second metal oxide thin film 130 may include at least two metal oxides selected from the same group, wherein one metal oxide acts as a matrix and supersaturates the other metal oxide to precipitate as particles . In this case, the size of the particles is in the range of 50 to 400 nm, and a minimum thickness is required to at least contain the particles.

As shown in the figure, the second metal oxide thin film 130 formed on the lower surface of the base substrate 110 is used as an internal light extraction layer of the OLED. The second metal oxide thin film 130 formed as an internal light extraction layer in this manner has a haze value of 5% or more, a transmittance of 50% or more in the visible light range, and a PTFE in the range of 1.4 to 3.0. refractive index.

The third metal oxide film 140 is formed such that it coats the surface of the second metal oxide film 130 . It is preferable that the thickness of the third metal oxide thin film 140 is in the range of 50 to 2000 nm. The third metal oxide thin film 140 may include a metal oxide, a solid solution of at least two metal oxides, or a multilayer structure of at least two metal oxides, wherein the metal oxide is selected from ZnO, SnO ₂ , SiO ₂ , In the group consisting of Al ₂ O ₃ and TiO ₂ . Here, since it is used as a transparent electrode of the OLED, the third metal oxide thin film 140 must have electrical properties. For this, the metal oxide may include at least one of an n-type dopant including Ga, Al, F, Si, and B and a p-type dopant including N. Therefore, the third metal oxide thin film 140 has a sheet resistance of 15Ω/□.

As shown in FIGS. 1 and 7, the third metal oxide thin film 140 forms a flat surface. Accordingly, the third metal oxide thin film 140 or the transparent electrode has a haze value of 10% or less. The transparent electrode has a transmittance of 70% or more in a visible light range, and a refractive index in a range of 1.7 to 3.0.

A method of manufacturing a substrate for an OLED according to an embodiment of the present invention will now be cited with reference to FIGS. 2 and 3 .

The method of manufacturing a substrate for an OLED according to this embodiment of the present invention deposits at least one metal oxide thin film on each of one and the other surfaces of a base substrate by APCVD. When a metal oxide thin film is formed by APCVD, textures are spontaneously formed on the surface of the metal oxide thin film during deposition of the thin film. That is, when the metal oxide thin film is formed by APCVD, the process of forming the texture can be omitted. This therefore simplifies the manufacturing process and improves yield, thereby enabling mass production.

The APCVD process includes loading a base substrate into a process chamber, and then heating the base substrate to a predetermined temperature. Then, a precursor gas and an oxidant gas are blown into the processing chamber to form a metal oxide film by APCVD. The feeding of the precursor gas and the oxidant gas along different paths is preferably controlled to prevent mixing of the gases prior to entering the processing chamber. Precursor and oxidant gases can be preheated prior to dosing to facilitate chemical reactions. A precursor gas may flow into the processing chamber in a carrier gas, which may be an inert gas such as nitrogen, helium or argon.

In the case of depositing a metal oxide thin film by APCVD, the surface of the base substrate may be modified by plasma treatment or chemical treatment before APCVD starts to control the shape of the texture formed on the surface of the metal oxide thin film. In addition, the surface of the metal oxide thin film may be modified by plasma or chemical treatment after APCVD to control the shape of the texture formed on the surface of the metal oxide thin film formed by APCVD.

As shown in FIG. 2 , a method of manufacturing a substrate for an OLED by APCVD includes: first, a step of preparing a base substrate 110 . Here, polymer materials such as thermally or ultraviolet (UV) cured organic films or such as soda lime glass (SiO ₂ -CaO—Na ₂ O) or aluminosilicate glass (SiO ₂ -Al ₂ O ₃ -Na ₂ O) The base substrate 110 is made of chemically tempered glass.

In turn, the first metal oxide thin film 120 serving as an external light extraction layer of the OLED is formed on the upper surface of the base substrate 110 by deposition. To deposit the metal oxide thin film 120, at least one substance selected from the group consisting of ZnO, _SnO2 , _SiO2 , _{Al2O3 ,} and _TiO2 of metal oxides or solid solutions thereof may be used. As described above, when the first metal oxide thin film 120 is deposited by APCVD, the first texture 120a is spontaneously formed on the surface of the first metal oxide thin film 120 .

Then, a second metal oxide thin film 130 serving as an external light extraction layer of the OLED is deposited on the lower surface of the base substrate 110 . To deposit the second metal oxide thin film 130, at least one substance selected from the group consisting of ZnO, _SnO2 , _SiO2 , _{Al2O3 and TiO2} of metal oxides or their solid solutions may be used. Also, when the second metal oxide thin film 130 is deposited by APCVD, the second texture 130a is spontaneously formed on the surface of the second metal oxide thin film 130 . Here, a planarization layer (not shown) is preferably formed on the surface of the second texture 130a to ensure the planarity of the third metal oxide film 140 to be formed in a later step.

The second metal oxide film 130 may include a metal oxide, a solid solution of at least two metal oxides, or a multilayer structure of at least two metal oxides, wherein the metal oxide is selected from ZnO, SnO ₂ , SiO ₂ , Al ₂ O ₃ and TiO ₂ in the group. As shown in part (a) of FIG. 2, the second metal oxide thin film 130 may be formed such that one substance acts as a matrix and the other substance is supersaturated and precipitated as particles 130b. In this case, the size of the particles 130b is in the range of 50 to 400 nm, and the matrix must have a minimum thickness so that the particles 130b can be formed therein.

Finally, a third metal oxide thin film 140 serving as a transparent electrode of the OLED is formed on the surface of the second metal oxide thin film 130 by deposition. In order to deposit the third metal oxide thin film 140, at least one substance selected from the group _consisting of ZnO, _SnO2 , _SiO2 , _Al2O3 , and _TiO2 of metal oxides or solid solutions thereof may be used. In addition, the third metal oxide film 140 may be treated with at least one of an n-type dopant including Ga, Al, F, Si, and B and a p-type dopant including N to make the third metal oxide film 140 conductive. .

When the third metal oxide thin film 140 is deposited in this way, the fabrication of the substrate for OLED according to this embodiment of the present invention is completed. In the method of manufacturing the substrate for OLED according to this embodiment of the present invention, the above steps are performed by direct insertion process based on APCVD. Therefore, the thin film deposition process performed by different processes in the prior art can be simplified, thereby reducing manufacturing costs and improving functional matching.

Alternatively, as shown in FIG. 3, the first metal thin film 120 may be deposited after forming the second and third metal oxide thin films 130 and 140 in advance.

The foregoing description of specific exemplary embodiments of the present invention has been expressed with reference to the accompanying drawings. They are not intended to be exhaustive, or to limit the invention to the precise form disclosed, but obviously many modifications and variations are possible to those skilled in the art in light of the above teachings.

Accordingly, the scope of the present invention is not intended to be limited to the above-described embodiments, but is defined by the appended claims and their equivalents.

Claims (17)

1. A substrate for an organic light-emitting device, comprising: base substrate; coating a first metal oxide film on one surface of the base substrate, the first metal oxide film having a first texture on its surface; coating the other surface of the base substrate with a second metal oxide thin film; and Coating the third metal oxide thin film on the surface of the second metal oxide thin film. 2. The substrate according to claim 1, wherein the first metal oxide thin film constitutes an external light extraction layer of the organic light emitting device, and the second metal oxide thin film constitutes an internal light extraction layer of the organic light emitting device. extraction layer, and the third metal oxide thin film constitutes a transparent electrode of the organic light-emitting device. 3. The substrate of claim 1, wherein the second metal oxide thin film has a second texture on its surface, wherein the surface is coated with the third metal oxide thin film. 4. The substrate of claim 3, further comprising a planarization layer between the second metal oxide thin film and the third metal oxide thin film. 5. The substrate of claim 1, wherein a surface of the second metal oxide thin film coated with the third metal oxide thin film forms a flat surface. 6. The substrate according to claim 1, wherein each of the first to third metal oxide thin films comprises a metal oxide, a solid solution of at least two metal oxides, or a mixture of at least two metal oxides. A multilayer structure, wherein the metal oxide is selected from the group consisting of ZnO, SnO ₂ , SiO ₂ , Al ₂ O ₃ and TiO ₂ . 7. The substrate according to claim 2, wherein the haze value of the outer light extraction layer is 60% or more, the haze value of the inner light extraction layer is 5% or more, and the The haze value of the transparent electrode is 10% or less. 8. The substrate according to claim 2, wherein the transparent electrode has a sheet resistance of 15Ω/□ or less. 9. The substrate of claim 2, wherein the outer light extraction layer has a transmittance of 40% or more and the inner light extraction layer has a transmittance of 50% or more in the range of visible light , and the transmittance of the transparent electrode is 70% or greater. 10. The substrate of claim 2, wherein the outer light extraction layer has a refractive index in the range of 1.4 to 3.0, the inner light extraction layer has a refractive index in the range of 1.4 to 3.0, and the transparent The refractive index of the electrodes is in the range of 1.7 to 3.0. 11. A method of manufacturing a substrate for an organic light emitting device, comprising depositing at least one metal oxide thin film on each of one surface and the other surface of a base substrate by atmospheric pressure chemical vapor deposition. 12. The method of claim 11, wherein depositing the at least one metal oxide film comprises: depositing a first metal oxide thin film on one surface of the base substrate as an external light extraction layer of the organic light emitting device; depositing a second metal oxide thin film on the other surface of the base substrate as an internal light extraction layer of the organic light emitting device; and A third metal oxide thin film is deposited on the surface of the second metal oxide thin film as a transparent electrode of the organic light emitting device. 13. The method according to claim 12, wherein the deposition of the second metal oxide film and the deposition of the third metal oxide film are performed after depositing the first metal oxide film, or after depositing After the second metal oxide thin film and the third metal oxide thin film are deposited, the first metal oxide thin film is deposited. 14. The method of claim 12, further comprising forming a planarization layer on the second metal oxide film between depositing the second metal oxide film and depositing the third metal oxide film . 15. The method of claim 14, wherein depositing the third metal oxide film comprises using at least one n-type dopant comprising Ga, Al, F, Si, and B or a p-type dopant comprising N. The dopant dopes the third metal oxide thin film. 16. The method of claim 12, wherein the deposition of the first metal oxide film, the deposition of the second metal oxide film and the deposition of the third metal oxide film are performed by direct insertion processing. deposition. 17. The method of claim 12, wherein each of the first to third metal oxide thin films is deposited with a metal oxide or a solid solution of at least two metal oxides, wherein the metal oxide is selected from In the group consisting of ZnO, SnO ₂ , SiO ₂ , Al ₂ O ₃ and TiO ₂ .

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